Synthesis and investigation on cobalt layered hydroxide compounds and their related nanostructured materials

i
Acknowledgements

On publication of my thesis, I am grateful for the contributions of several
individuals, without which this would have been impossible to achieve.
I am extremely indebted to my supervisor A/P Zeng Hua Chun for his
invaluable direction, advice and encouragement throughout the duration of this
research. His endless patience and understanding has allowed me to carry out this
work to the best of my ability.
I would sincerely like to thank Dr Xu Zhiping and Dr Gowry Sampanthar for
many useful discussions and their assistance in carrying out research work in many
aspects.
For technical support, I am especially grateful to Dr Li Sheng and Mdm Sam
Fam Hwee Koong for XPS, Mr Chia Phai Ann and Mr Mao Ning for TEM. Many
thanks go to Ms Lee Chai Keng, Mdm Khoh Leng Khim and Ms Tay Choon Yen for
their support manytimes in running other instruments.
Special thanks must go to my husband, my parents and Ms Zhang Yingsu and
her daughter Mao Bangyuan for their unfailing support, encouragement and
understanding during the last three years.

ii
Table of Content


Acknowledgements i
Summary vi
Nomenclature viii
List of Figures xi
List of Tables xvi
Publications Related to Thesis xviii

Chapter 1 Scope of the Thesis 1
Chapter 2 Literature Review 7
2.1 Overview 7
2.2 Layered Hydroxide Compounds 8
2.2.1 Introduction 8
2.2.2 Structural properties 9
2.2.2.1 The nature of M
II
and M
III
11
2.2.2.2 The values of x 13
2.2.2.3 Nature of anions 14
2.2.2.4 The values of m 16
2.2.3 Synthesis methods 17
2.2.3.1 Coprecipitation method 18
2.2.3.2 Anion exchange method 19
2.2.3.3 Rehydration method 22
2.2.3.4 Hydrothermal synthesis 22
2.2.3.5 Sol-gel method 23
2.2.3.6 Oxidation method 24
2.2.3.7 Microwave assisted method 24
2.2.4 Applications 25
2.2.4.1 N
2
O decomposition 26
2.2.4.2 Catalytic partial oxidation (CPO) of methane 27
2.2.4.3 Steam reforming 27
2.2.4.4 Polymerization reactions 28
2.2.4.5 Aldol condensation 28

2.2.4.6 Hydrogenation of nitriles 29
2.2.4.7 Oxidation 30
2.2.4.8 Catalyst supports 31
2.2.4.9 Anion adsorbents 32
2.3 Cobalt-Layered Hydroxides 33

iii
2.3.1 Introduction 33
2.3.2 Cobalt hydroxides 33
2.3.3 Cobalt-containing HTlcs 34
2.3.4 Cobalt-containing hydroxide salts 35
2.4 Nanostructured Materials 35
2.4.1 Introduction 35
2.4.2 Types of nanostructured materials 36
2.4.3 Synthesis of nanostructured materials 37
2.4.4 Chemical synthesis of Co-layered hydroxides and the related
nanostructured materials 41
Chapter 3 Experimental Methods 46
3.1 Materials Preparation 46
3.2 Characterization Methods 46
3.2.1 Powder X-ray diffraction (XRD) 46
3.2.2 Fourier transform infrared spectroscopy (FTIR) 47
3.2.3 Thermogravimetry analysis (TGA) 48
3.2.4 Thermogravimetry analysis-Fourier transform infrared spectroscopy
(TGA-FTIR) 48
3.2.5 X-ray photoelectron spectroscopy (XPS) 49
3.2.6 Transmission electron microscopy (TEM) 50
3.2.7 Selected area electron diffraction (SAED) 50
3.2.8 Inductive coupled plasma –Atomic emission spectroscopy (ICP-AES) 51
3.2.9 Determination of carbon and nitrogen 51

Chapter 4 A Comparative X-ray Photoelectron Spectroscopic
Investigation on Cobalt-Containing Layered Hydroxides
52
4.1 Introduction 52
4.2 Material Synthesis and Characterization 53
4.3 Results and Discussion 55
4.3.1 Overall bulk and surface compositional analyses 55
4.3.2 Surface adsorbed species and intercalated anions 57
4.3.3 Hydroxyl and other oxygen species 61
4.3.4 Trivalent metal cations in brucite-like layers 67
4.3.5 Divalent metal cations in brucite-like layers 70
4.4 Conclusions 78

iv
Chapter 5 Mechanistic Investigation of Self-Redox
Decomposition of Cobalt-Hydroxide-Nitrate Compounds with
Different Nitrate Anion Configurations in Interlayer Space
80
5.1 Introduction 80
5.2 Material Synthesis and Characterization 81
5.3 Results and Discussion 83
5.3.1 Structural and chemical analysis 83
5.3.2 Configurations of nitrate anions in interlayer space 86
5.3.3 Self-redox decomposition of intercalated nitrate anions 93
5.3.4 Surface analysis of decomposed products 99
5.4 Conclusions 106
Chapter 6 Investigation on the Effect of Molecular Symmetry
on Decomposition Processes of PA/TA-Intercalated Layered
Hydroxides
108

6.1 Introduction 108
6.2 Material Synthesis and Characterization 109
6.3 Results and Discussion 111
6.3.1 Structure and composition of Co(Mg)Al-TA(PA)-HTlcs 111
6.3.2 Thermal decomposition 117
6.3.3 Formation of metal-oxide/carbon nanocomposites 126
6.4 Conclusions 137
Chapter 7 Mechanistic Investigation on Salt-Mediated
Formation of Free-Standing Co
3
O
4
Nanocubes at 95 °C from
Cobalt-Layered Hydroxide Precursors
138
7.1 Introduction 138
7.2 Material Snthesis and Caracterization 139
7.3 Results and Discussion 140
7.3.1 Composition and structural phases 140
7.3.2 Thermal decomposition 148
7.3.3 Cystallite morphologies 152
7.4 Conclusions 162
Chapter 8 Preparation of Nanorods of Cobalt Hydroxide
Carbonate and the Derived Co
3
O
4
One-Dimensional
Nanostructures
163

8.1 Introduction 163

v
8.2 Material Synthesis and Characterization 165
8.3 Results and Discussion 166
8.3.1 Composition and structural phases 166
8.3.2 Crystallite microstructures 176
8.4 Conclusions 188
Chapter 9 Concluding Remarks and Suggestions 189
9.1 Concluding remarks 189
9.2 Suggestions for future work 191
References 193

vi
Summary


The research work carried out in this thesis is related to cobalt-containing
layered hydroxide compounds and their derived nanostructured materials, due to the
growing interest in new applications of such materials as catalysts, organic-inorganic
hybrid nanocomposites and nanostructured materials. The element cobalt was chosen
as it is an important transition metal having common oxidation states of +2 and +3,
which gives interesting properties to the derived compounds, in the form of hydroxide
and oxide. The layered materials studied in this thesis include hydrotalcite-like
compounds and hydroxide salts. The basic structure of such compounds consists of
metal hydroxide layers and anions intercalated or pillared into the layer structures.
Chapter 2 gives a literature review about such materials regarding their
physicochemical properties, synthesis methods and applications. In addition, some
introduction specifically on cobalt-containing layered hydroxide materials is provided.
In this chapter, the nanostructured materials are also briefly introduced and the

synthesis of cobalt-layered hydroxides and the related nanostructured materials is
summarized. Chapter 3 is an introduction to the experimental methods involved in this
work. It presents the methods for materials preparation and characterization
techniques.
Chapter 4 is concerned with surface chemical states of cobalt-containing layered
hydroxides, as such information is rare in open literature despite that the bulk information
of these materials have been well documented. The spectra obtained with XPS for
various elements indicate the composition and bonding environment in the surface region.
The effects of metals, population and valency of interlayer anions are observed and
discussed.

vii
In Chapters 5 and 6, investigation is carried out on decomposition processes of
cobalt-layered compounds intercalated with anions. Chapter 5 studies in detail about the
mechanism of the self-redox decomposition of two nitrate-containing cobalt layered
hydroxide compounds, with variable redox reagent contents and different
configurations of nitrate anions in the interlayer space. It has been elucidated that
divalent cations in resulting oxides, rather than in hydroxyl octahedral, are the active
reductant for the redox reactions. Chapter 6 presents the effect of molecular symmetry
of the intercalated organic bi-carboxylate anions on the decomposition process. In this
study, terephthalate (TA
2-
) and 1,2-phenylenediacetic (PA
2-
) anions have been
intercalated into CoAl- and MgAl-hydrotalcite-like compounds. It has been found that
within the same confined space and chemical environment provided by the hydroxide
layers, decomposition processes of PA
2-
-containing HTlcs are distinctively different

from those of TA
2-
-containing HTlcs due to the variations in molecular structure and
charge distribution.
Chapters 7 and 8 are focused on synthesis of cobalt spinel oxide, Co
3
O
4

nanocrystals. The generation of free-standing Co
3
O
4
nanocubes of ca. 47 nm via
increasing of the ionic strength in aqueous system at 95
o
C and atmospheric pressure is
described in Chapter 7. The formation of nanosized and faceted nanocubes is found to
be attributed to the reduction of interfacial tension and the creation of salt-(solvent)
n

diffusion boundary in the solution with high ionic strength. In Chapter 8, the effort has
been made to prepare Co
3
O
4
nanocrystals assembled in rod-like shape via thermal
transformation from rod-like cobalt hydroxide carbonate precursors.
Finally, Chapter 9 gives conclusions about the proceeding chapters and some
suggestions for the future work.

viii
Nomenclature



a unit cell in HTlc (inter-metal-distance in brucite-like layers)
A′′2 symbol of IR vibrational symmetry
AC adventitious carbon
AES atomic emission spectroscopy
BE binding energy (eV)
BEs binding energies (eV)
BET Brunauer-Emmett-Teller method
c′ inter-brucite-like-sheet-distance
c 3c′ or 2c′ (unit cell parameter in c-direction)
C
2v
IR vibrational symmetry with only one double axis
CHN elemental analysis of carbon, hydrogen and nitrogen
CPO catalytic partial oxidation
C
x
H
y
unsaturated hydrocarbon compounds
d distance between two planes
δ bending vibration
D diameter of ring in electron diffraction pattern

D

3h
symmetry with one triple axis and three perpendicular double axes
d
hkl
distance between reflection planes (hkl)
Dia. diameter of γ-alumina pellets (mm)
DNA deoxyribonucleic acid
D
p
average crystalline size (nm)
DrTGA differential thermogravimetry analysis
E′ ν
3
vibrational mode of D
3h

ix
ED electron diffraction
Eq. equation
Eqs. equations
EXAFS extended X-ray absorption fine struture
φ specimen tilted angle in transmission electron microscope analysis
Fig. figure
Figs. figures
FTIR Fourier transform infrared spectroscopy
FWHM full width at half maximum
h hour(s)
H height of γ-alumina pellets (mm)
HC hydrocarbon
HRTEM high resolution transmission electron microscope

HT hydrotalcite
HTlc hydrotalcite-like compound
HTlcs hydrotalcite-like compounds
ICP inductively coupled plasma
λ wavelength of X-ray radiation (0.1506 nm for Cu Kα radiation)
λ
e
wavelength of electron beam
m the amount of interlayer water in hydrotalcite-like compounds
ν
n
symbol of IR vibrational mode
ν
as
asymmetric stretching vibration
ν
s
symmetric stretching vibration
N number of sites occupied by anions in interlayer space
OC oxidized carbon

x
PA 1,2-phenylenediacetic acid
PA
2-
1,2-phenylenediactate anion
θ diffraction angle in the X-ray diffraction measurements (
o
)
ss shake-up satellite

SAED selected area electron diffraction
T temperature
TA terephthalic acid
TA
2-
terephthalate anion
TEM transmission electron microscope
TGA thermogravimetry analysis
vpm volume per million
XPS X-ray photoelectron spectroscopy
XRD X-ray diffraction





xi
List of Figures


Figure 2.1


Figure 4.1

Figure 4.2

Figure 4.3

Figure 4.4


Figure 4.5







Figure 5.1






Figure 5.2



Figure 5.3

Figure 5.4
Schematic diagrams for (a) brucite lattice; (b)hydrotalcite
lattice; and (c) atom composition.

XPS spectra of C 1s for all the samples listed in Table 4.1.

XPS spectra of O 1s for all the samples listed in Table 4.1.

XPS spectra of Al 2p for all the aluminium-containing samples
listed in Table 4.1.

XPS spectra of Co 2p
3/2
and Co 2p
1/2
for all the cobalt-
containing samples listed in Table 4.1.

A proposed surface structure for anion grafting (Peak 2, Table
4.6) on external surface of brucite-like layers (Peak 1, Table
4.6); the light-gray areas indicate the brucite-like or
hydrotalcite-like layered structures while the darkened areas
represent for anion-grafted brucite-like layers or for cobaltous
salts (nitrate and carbonate) in which anion oxygen is directly

linked to cobalt.

XRD patterns of two cobalt-hydroxide-nitrate compounds
Co
II,III
-0.2 and Co
II
-0.5. Reflections of polycrystalline silicon
(internal standard) are indicated with asterisk symbol. Insert
indicates an alternate arrangement of oxidant and reductant in
the two compounds and inter-brucite-like-sheet-distance
assignment under two different crystal symmetries.

FTIR spectra of as-prepared cobalt-hydroxide-nitrate
compounds Co
II,III
-0.2 and Co
II
-0.5 using KBr-pellet and
Nujol-mull techniques.

Nitrate anion configurations in interlayer space for a Co-
hydrotalcite-like compound with 1/3 of total cobalt cations in
trivalent oxidation state. In model I, every oxygen atom has
two hydroxyl groups respectively from two adjacent brucite-
like layers (above and below, marked with 1, 2 and 3). In
model II, oxygen atoms are not connected directly to hydroxyl
groups (marked with 1, 2 and 3) from the two adjacent brucite-
like layers in the normal HT stacking sequence, but they may
be connected to hydroxyl groups directly when a turbostratic

stacking occurs in the upper layer. Arrows indicate the
vibrational directions of atoms in asymmetric stretch mode
(ν
3
), while positive and negative signs show the out-of-plane
bending mode (ν
2
).

Nitrate anion configurations in interlayer space for a Co-

9

59

63


69


73







76

84



87













89



xii

Figure 5.5



Figure 5.6




Figure 5.7





Figure 5.8



Figure 5.9

Figure 5.10



Figure 6.1


Figure 6.2


Figure 6.3



Figure 6.4



Figure 6.5
hydroxide-nitrate compound in which all cobalt cations are in
divalent oxidation state. Starting from a hydroxyl vacancy, on
one hand, a nitrate anion can be incorporated into the brucite-
like plane through one of its three oxygen atoms (C
2v
; model
I), which leads to a modification of hydroxyl sublattice. On
the other hand, a nitrate anion could directly attach to the same
vacancy with a D
3h
site-symmetry (model II).

TGA and DrTGA scans for as-prepared Co
II,III
-0.2 and Co
II
-0.5
compounds heated in nitrogen atmosphere; gas flowrate 100
mL min
-1
.

Integrated absorbances of evolved gases NO
2
, H
2
O and CO
2

versus heating temperature in the combined TGA-FTIR
measurements for the samples of (a) Co
II,III
-0.2, and (b) Co
II
-
0.5; nitrogen flowrate = 100 mL min
-1
.

XRD patterns of decomposed Co
II,III

-0.2 and Co
II
-0.5
compounds. The two samples were heated from room
temperature to 400
o
C in nitrogen with TGA furnace (at 10
o
C
min
-1
), then brought back to room temperature in the same
inert atmosphere; gas flowrate = 100 mL min
-1
.

O 1s photoelectron spectra of Co
II,III
-0.2 and Co
II
-0.5
compounds as well as their respective samples heated over the
temperature range of 220-400
o
C.

N 1s photoelectron spectra of Co
II,III
-0.2 and Co
II

-0.5
compounds as well as their respective samples heated over the
temperature range of 220-400
o
C.

Co 2p
3/2
and Co 2p
1/2
photoelectron spectra of Co
II,III
-0.2 and
Co
II
-0.5 compounds as well as their respective samples heated
over the temperature range of 220-400
o
C.

XRD patterns of the as-prepared TA or PA containing
hydrotalcite-like samples CoTA, MgTA, CoPA and MgPA.

Two possible ways of molecular arrangement of PA in the
interlayer space of hydrotalcite-like materials.

FTIR spectra of the as-prepared TA or PA containing
hydrotalcite-like samples CoTA, CoPA, CoTA(PA), MgTA,
MgPA and MgTA(PA).

Decomposition of the as-prepared CoTA, MgTA, CoPA and
MgPA samples in air: (a) TGA curves and (b) DrTGA curves
(heating rate: 10 and 2
o
C min
-1
; flow-rate: 40 mL min
-1
).

Decomposition of the as-prepared CoTA, MgTA, CoPA and






92



94




95

98



100



103



105


112


113



115



118

xiii




Figure 6.6




Figure 6.7a


Figure 6.7b


Figure 6.7c


Figure 6.7d


Figure 6.8a-f

Figure 6.8g-l





Figure 6.9

Figure 7.1a




Figure 7.1b




Figure 7.2a


Figure 7.2b

MgPA samples in nitrogen: (a) TGA curves and (b) DrTGA
curves (heating rate: 10 and 2
o
C min
-1
; flow-rate: 100 mL
min

-1
).

Chromatogram of C
x
H
y
in the combined TGA/FTIR
measurements for samples CoTA, MgTA, CoPA and MgPA
with nitrogen as carrier gas (heating rate: 10
o
C min
-1
; flow-
rate: 100 mL min
-1
).

FTIR spectra of calcined CoTA in nitrogen at 400, 450 and
500
o
C with heating rate of 10, 2 and 0.4
o
C min
-1
.

FTIR spectra of calcined CoPA in nitrogen at 400 and 450
o
C

with heating rate of 10, 2 and 0.4
o
C min
-1
.

FTIR spectra of calcined MgTA in nitrogen at 500 and 550
o
C
with heating rate of 10, 2 and 0.4
o
C min
-1
.

FTIR spectra of calcined MgPA in nitrogen at 500 and 550
o
C
with heating rate of 10, 2 and 0.4
o
C min
-1
.

TEM pictures of calcined samples in nitrogen for 2 h: (a)
CoTA (10
o
C min
-1
; 500

o
C); (b) CoTA (2
o
C min
-1
; 500
o
C);
(c) CoTA (0.4
o
C min
-1
; 500
o
C); (d) MgTA (10
o
C min
-1
; 550
o
C); (e) MgTA (2
o
C min
-1
; 550
o
C); and (f) MgTA (0.4
o
C
min

-1
; 550
o
C).

TEM pictures of calcined samples in nitrogen for 2 h: (g)
CoPA (10
o
C min
-1
; 450
o
C); (h) CoPA (2
o
C min
-1
; 450
o
C);
(i) CoPA (0.4
o
C min
-1
; 450
o
C); (j) MgPA (10
o
C min
-1
; 550

o
C); (k) MgPA (2
o
C min
-1
; 550
o
C); and (l) MgPA (0.4
o
C
min
-1
; 550
o
C).

XRD curves of calcined samples in nitrogen for 2 h.

XRD pattern evolution for the A series samples after 1.5-36 h
(i.e., samples A1.5 to A36) reactions (with 90 g of NaNO
3
) at
95
o
C: B = β-Co(OH)
2
; N = Co
II
(OH)
2-x

(NO
3
)
x
.
nH
2
O; HT =
Co
1-x
II
Co
x
III
(OH)
2
(NO
3
)
x
.
nH
2
O ; S = Co
3
O
4
phase.

XRD pattern evolution for the B series samples after 1.5-24 h

(i.e., samples B1.5 to B24) reactions (without using NaNO
3
) at
95
o
C: B = β-Co(OH)
2
; HT = Co
1-x
II
Co
x
III
(OH)
2
(NO
3
)
x
.
nH
2
O ;
S = Co
3
O
4
phase.

FTIR spectra for the A series samples after 1.5-36 h (i.e.,

samples A1.5 to A36) reactions (with 90 g of NaNO
3
) at 95
o
C.

FTIR spectra for the B series samples after 1.5-36 h (i.e.,
samples B1.5 to B24) reactions (without using NaNO
3
) at 95


122




125


129


130


131


132

134





135

136




142




143


145

xiv




Figure 7.3a



Figure 7.3b



Figure 7.4



Figure 7.5




Figure 7.6



Figure 7.7

Figure 7.8






Figure 7.9



Figure 8.1a



Figure 8.1b



Figure 8.2

o
C. FTIR spectra for the B series samples after 1.5-36 h (i.e.,
samples B1.5 to B24) reactions (without using NaNO
3
) at 95
o
C.

DrTGA curves for the A series samples (samples A1.5 to A36)
in nitrogen (heating rate: 10
o
C min
-1
; flow-rate: 100 mL
min
-1
).

DrTGA curves for the B series samples (samples B1.5 to B24)
in nitrogen (heating rate: 10
o
C min
-1
; flow-rate: 100 mL
min
-1
).

Comparison of SEM images of samples formed after reactions
at 95
o
C for (a) 6 and (b) 36 h with NaNO
3
; (c) 3 and (d) 24 h
without NaNO
3
.

TEM images with different magnifications for the Co
3
O
4

nanocubes formed after reactions at 95
o
C for (a) 36 h and (b)
48 h with NaNO
3
. The lower-right insert shows the lattice
fringes with interplanar distance d
111
.

TEM confirmation of cubic morphology with different tilted
angles (φ) for the Co
3
O
4
nanocubes formed after 48 h reactions
at 95
o
C with NaNO
3
.

TEM images with different magnifications (a and b) or the
Co
3

O
4
crystallites formed after reactions at 95
o
C for 24 h
(sample B24) without using NaNO
3
.

A reaction intermediate mixture of
Co
1-x
II
Co
x
III
(OH)
2
(NO
3
)
x
.
nH
2
O and Co
3
O
4
nanocubes (TEM

images for the sample after 21 h reactions at 95
o
C with
NaNO
3
): (a) precipitate mixture; (b) an enlarged part in panel
a; (c) Co
3
O
4
nanocube crystal interface. The interface is
indicated with arrows.

TEM images with different magnifications for the Co
3
O
4

nanocubes formed after reactions at 95
o
C for (a) 78 and (b) 96
h with NaNO
3
.

XRD patterns of as-prepared cobalt hydroxide carbonate
compounds, A1-A5, by heterogeneous precipitation method
using NaOH and Na
2
CO

3
.

XRD patterns of as-prepared cobalt hydroxide carbonate
compounds, B1-B4, by homogeneous precipitation method
using urea.

FTIR spectra of some representative as-prepared samples A5,
B2 and B4.


146



150



151



153




155

156



157






159



161



167



170


172

xv

Figure 8.3a



Figure 8.3b



Figure 8.4a



Figure 8.4b



Figure 8.4c



Figure 8.4d




Figure 8.5

Figure 8.6a



Figure 8.6b

TGA and DrTGA curves of samples A1-A5 (prepared by
heterogeneous precipitation method using NaOH and
Na
2
CO
3
), heating rate: 10
o
C min
-1
; air flow: 100 mL min
-1
.

TGA and DrTGA curves of samples B1-B4 (prepared by
homogeneous precipitation method using urea), heating rate:
10
o
C min
-1
; air flow: 100 mL min
-1

.

TEM pictures for samples A1-A5 (prepared by heterogeneous
precipitation method using NaOH and Na
2
CO
3
) and SAED
pattern for sample A5 (insert).

TEM pictures for samples B1-B4 (prepared by homogeneous
precipitation method using urea) and the free-standing rod for
each sample (insert).

SAED pattern of a rod selected from sample B4 (prepared by
homogeneous precipitation method using urea at 95
o
C for 20
h).

SAED patterns of the three different parts (two ends and the
middle) of another rod selected from sample B4 (prepared by
homogeneous precipitation method using urea at 95
o
C for 20
h).

XRD patterns of calcined samples in static air, B1-300
(heating rate: 2
o

C min
-1
; calcination temperature and time:
300
o
C and 3h) and B3-400 (heating rate: 2
o
C min
-1
;
calcination temperature and time: 400
o
C and 3h).

TEM and HRTEM pictures of calcined samples at respective
temperatures, 2
o
C min
-1
, 3 h, (a) B1-300, 300
o
C; (b) B1-400,
400
o
C ; (c) B1-500 500
o
C and (d) B3-400, 400
o
C.

(a) and (c) TEM images of sample B1-400, 400
o
C, 2
o
C min
-1
,
3h; (b) SAED pattern of (a); (d) SAED pattern of (c).



174



175



178



179



181

182




185



186


187


xvi
List of Tables


Table 2.1

Table 2.2

Table 2.3

Table 2.4

Table 2.5

Table 4.1


Table 4.2


Table 4.3



Table 4.4



Table 4.5



Table 4.6


Table 5.1


Table 5.2


Table 6.1

Table 6.2


Table 6.3


The ionic radii of some divalent and trivalent cations.

HTlcs obtained with different cations.

The values of c′ for some HTlcs.

Various formulas to determine the maximum value of m.

Organic anions incorporated to HTlcs using anion exchange
method.

Chemical formulas of the samples investigated with XPS
method.

Relative surface atomic compositions of C, O, Al and Co in
the solid compounds.

Binding energies (eV) of C 1s of different surface species and
their relative percentage atomic ratios (indicated in
parenthesis).

Binding energies (eV) of O 1s of different chemical species

and their relative contents (indicated in parenthesis) with
respect to Co (as unity).

Binding energies (eV) of Al 2p of different components and
their relative contents (indicated in parenthesis) to Co (as
unity).

Binding energies (eV) of Co 2p
3/2
of two different surface
components.

Results of compositional analysis and structural analysis for
samples Co
II,III
-0.2 and Co
II
-0.5.

Results of N 1s and Co 2p spectrum analyses (see Figures 5.9
and 5.10).

The initial solution composition and the estimated inter-
brucite-like-sheet distances and the average crystallite sizes.

The measured chemical composition and molecular formulas
of the as-prepared HTlcs.

The weight percentage of carbon in calcined samples with
various heating rate and temperatures.

12

12

15

16


21


55


56



57



62



68

72


85


104


110


117


128


xvii
Table 7.1


Table 8.1

Table 8.2
Samples prepared in this experiment with/without nitrate salt
and with different aging time.

Samples nomenclature and experimental conditions.

TGA and elemental analysis results for as-prepared cobalt
hydroxide carbonates samples.


140

166


176

xviii
Publications Related to Thesis


1. Xu, R. and Zeng, H. C. Synthesis of nanosize supported hydrotalcite-like
compounds CoAl
x
(OH)
2+2x
(CO
3
)
y
(NO
3
)
x-2y
.
nH
2
O on gamma- Al
2
O
3
, Chem.

Mater., 13, 297-303. 2001.

2. Xu, Z. P., Xu, R. and Zeng, H. C. Sulfate-functionalized carbon/metal-oxide
nanocomposites from hydrotalcite-like compounds, Nano Lett., 1, 703-706. 2001.

3. Xu, R. and Zeng, H. C. Mechanistic investigation on salt-mediated formation of
free-standing Co
3
O
4
nanocubes at 95
o
C, J. Phys. Chem. B, 107, 926-930. 2003.

4. Xu, R. and Zeng, H. C. Mechanistic investigation on self-redox decompositions
of cobalt-hydroxide-nitrate compounds with different nitrate anion configurations
in interlayer space, Chem. Mater., in press.

5. Xu, R. and Zeng, H. C. Investigation on the effect of molecular symmetry on
decomposition processes of PA/TA-intercalated clays, in preparation.

6. Xu, R. and Zeng, H. C. Preparation of nanorods of cobalt hydroxide carbonate
and the derived Co
3
O
3
one-dimensional nanostructures, in preparation.

1
Chapter 1 Scope of the Thesis



This thesis is concerning the cobalt layered hydroxides and their derived
nanostructured materials, mainly cobalt spinel oxides. Cobalt layered hydroxide is
used as a broad name for several types of compounds in this work. Besides the
relatively simple compound, Co(OH)
2
, the current work has been more focused on the
other two types of cobalt layered hydroxides incorporated with anions, namely,
hydrotalcite-like compounds and hydroxide salts or basic salts. In general, layered
hydroxide consists of two-dimensional hydroxide sheets stacking one on top of each
other. The sheet is formed by edge-sharing octahedra. In each octahedra, metal cation
is located in the center and surrounded by 6 OH
-
groups. If a fraction of Co
2+
ions is
replaced by trivalent cations (Co
3+
, or other trivalent cations), a net positive charge is

resulted in the hydroxyl sheet. This positive charge is balanced by exchangeable
interlayer anions. Such type of compounds is named as hydrotalcite-like materials.
Alternatively, if OH
-
vacancies are present in the hydroxyl sheet, the resulting net
positive charges are balanced by the interlayer anions which directly coordinate to the
metal group. Depending on the nature of cations and anions, they may or may not
occupy the interlayer space and such compounds in general is called hydroxide salt. In
the current research, four types of work have been involved:
1) Synthesis of cobalt layered hydroxide materials mainly by coprecipitation
method;
2) Characterization of the above materials by XRD/FTIR/TGA/CHN/ICP/XPS,
etc.;
3) Detailed investigation on their thermal decomposition behavior by in situ TGA-
FTIR experiment coupled with other characterization methods listed in 2);

2
4) Synthesis of cobalt spinel oxide nanomaterials from cobalt layered hydroxide
precursor compounds under oxidative conditions.
First of all, some effort was made in Chapter 2 to give an overall introduction
on layered hydroxide materials regarding their general physicochemical properties,
synthesis methods and applications, as well as nanostructured materials and in
particular nanomaterials of cobalt layered hydroxides and their related compounds,
mainly cobalt spinel oxide.
Chapter 3 intends to highlight the general experimental synthesis techniques
and the principles of all the instrumental methods used in the present thesis work.
The work carried out in Chapter 4 presents a detailed investigation on surface
chemical states of Co-containing layered hydroxides by X-ray photoelectron
spectroscopy (XPS). Over the past decades, extensive chemical and physical
characterization work has been conducted for obtaining the bulk information of layered

hydroxides, such as crystallographic structure, chemical bonding, thermal behavior and
chemical composition. However, the surface chemistry of such materials has not
received much attention. The samples investigated which were readily synthesized in
our lab included both single-phase and γ-Al
2
O
3
supported CoAl-HTlcs intercalated
with carbonate and/or nitrate anions. Other commercial available compounds,
Co(OH)
2
, CoCO
3
⋅xH
2
O, Co(NO
3
)
2
and γ-Al
2
O
3
, were also analyzed to provide
complementary information. XPS analysis in the surface region was conducted for C
1s, O 1s, Co 2p and Al 2p. The chemical species and bonding information were
deduced based on the binding energies from the deconvoluted peaks for all the
elements. The relative atomic compositions on the surface were estimated by
quantitative treatment of the XPS spectra obtained. The results demonstrated that XPS

3
can be used as a very sensitive technique for the elucidation of surface information of
Co-containing layered hydroxides in great detail.
After being familiarized with XPS technique and obtaining a sound
understanding about the surface chemistry, the author carried out further research on
thermal decomposition behavior of cobalt-containing layered hydroxides, which is
presented in Chapters 5 and 6. The chemical reactions under thermal conditions
involve both brucite-like sheets and the intercalated anions. The former decompose
into catalytically active metal oxides, which then accelerate the reactions of the latter.
Such reactions are worth studying as the reactants (the metal oxides and the interlayer
anions) are stacked in alternate layers with molecular level mixing. In other words, the
surface contact between different species is maximized and the distance that the
reactants need to diffuse is minimized. Interesting results can be obtained especially
when the reactions involve reactants of different redox reactivity and anions of
different configurations and symmetries in the interlayer space.
In Chapter 5, such experimental findings are presented by the studies on self-
redox decompositions of two nitrate containing cobalt layered hydroxide compounds,
Co
II
0.80
Co
III
0.20
(OH)
2.00
(NO
3
)
0.14
(CO

3
)
0.03
⋅0.77H
2
O and Co
II
(OH)
1.50
(NO
3
)
0.40
(CO
3
)
0.05
⋅0.05H
2
O under inert atmosphere condition. These two compounds were purposely
prepared with variable redox reagent contents and different configurations of nitrate
anions in the interlayer space. Based on the detailed investigation using TGA-FTIR
and XPS methods, the decomposition temperatures, sequence of reactions and gas-
evolving patterns of the two samples were investigated for a mechanistic
understanding of the self-redox decompositions of cobalt-containing layered
hydroxides. The final decomposition products of both compounds are all in nanophase
Co
3
O
4

.

4
Chapter 6 extends the thermal studies to organic anions-intercalated CoAl- and
MgAl-layered hydroxides. Such organic-inorganic composite materials have potential
applications in newly emerging fields, such as organic-inorganic nanocomposites,
biomolecular-inorganic nanohybrids and precursors for nanostructured materials. To
achieve a better design of the precursor compounds, the chemical reactivity between
the intercalated organic anions and the metal cations needs to be addressed in detailed.
In this chapter, terephthlate and 1,2-phenylenediacetic anions have been intercalated
into CoAl-HTlcs and MgAl-HTlcs by the coprecipitation method. Due to the
difference in the molecular symmetries and coordination patterns of carbonate anions
on the benzene ring for these two anions, the physicochemical properties of the
precursors, such as the anion orientations, the particle sizes and anion intercalation
selectivity varied. All these properties in turn influenced the chemical reactivities of
metal groups and the organic anions, which were investigated by TGA-
FTIR/FTIR/CHN analysis. During the thermal reactions, the heating rate was varied
with the aim to control the morphological properties of the decomposed products.
The last part of the thesis work was focused on synthesis of cobalt spinel oxide,
Co
3
O
4
, by transformation of cobalt hydroxide-types of precursor materials under both
ambient condition in aqueous system (Chapter 7) and thermal condition (Chapter 8).
The aim was to obtain Co
3
O
4
nanoparticles with novel properties, which stands a new

field of interest, although extensive synthesis work has been carried out for such
materials over the past decades.
As presented in Chapter 7, cobalt layered hydroxides were first prepared with
precipitation method by adding cobalt nitrate aqueous solution into sodium hydroxide
solution, under oxidative condition with air purging at 95
o
C. The experiment was run
with salt mediation by addition of large amount of NaNO
3
into the initial sodium

5
hydroxide solution. During the continued aging in the mother liquor with air purging,
the precursor cobalt layered hydroxides gradually transformed to cobalt spinel oxide.
The effect of adding an electrolyte is to reduce the thickness of the counter-ion
“atmosphere” surrounding the particles, which then affect the coagulation or
aggregation of the primary particles into final products, monodispersed Co
3
O
4

nanocubes. As it was our objective to understand the formation mechanism of such
particles, the intermediate compounds were also fully characterized to “capture” the
transformation process by XRD/FTIR/TGA/TEM methods. The same experiment in
the absence of salt mediation was also conducted for comparison.
The effort to prepare Co
3
O
4
nanoparticles assembled in rod-like shape was

made and the results are reported in Chapter 8. The precursor materials, cobalt
hydroxide carbonate compounds, were synthesized by precipitation methods, either in
heterogeneous or homogeneous way. The addition of the solution containing NaOH
and Na
2
CO
3
into the solution of cobalt salt generated cobalt hydroxide carbonates in
the form of monodispersed nanorods. At the same time, the homogeneous
precipitation of cobalt salts in the presence of urea hydrolysis also led to the formation
of nanorods with larger size of the similar compounds. Combined TGA and CHN
analysis determined the molecular formula of such compounds. The as-formed
hydroxide carbonate compounds were then calcined under oxidative condition in order
to obtain Co
3
O
4
nanoparticles. The original rod-like morphology was partially
retained with interconnected sub-particles of cobalt spinel oxides after calcination.
The morphology could also be viewed as self-assembly of Co
3
O
4
nanoparticles in rod-
shape, obtained by relatively simple synthetic way compared to those conventional
self-assembly techniques. The microstructures of the pristine cobalt hydroxide
carbonates as well as the final Co
3
O
4

products were analyzed in detail in order to

6
disclose the crystalline orientation and the formation mechanism of rod-like
morphology via this simple precipitation method.
Finally, Chapter 9 briefly summarizes the major experimental results of the
current research work on cobalt layered hydroxides and the related nanostructural
materials. An attempt to suggest some future work has also been made at the end of
this chapter.


7
Chapter 2 Literature Review

2.1 Overview

Since the report of the novel synthesis method for carbon nanotubes in early
1990’s by Iijima,
1
world-wide ranging research activities have been put into the
development nanostructured materials which can be used in the areas of
semiconductor, biomedical and catalysis, etc. There are various newly emerged
methods in the literature for the preparations of the nanosized particles, such as carbon
arc-discharge, electron beam irradiation, laser ablation,
2-6
catalytic decomposition,
7

hydrothermal technique,
8-10

sol-gel method,
11
surfactant-assisted templating method,
12-
14
etc. These methods are all effective in one way or another towards the generation of
nanostructured materials with desired properties. Compared to these methods,
conventional ambient-temperature precipitation methods are less explored. The main
reason is that such simple methods normally produce relatively large particles with
varied grain sizes in the scale of microns and above. However, these are usually the
simpler and less costly way of performing synthesis. In order to achieve particle size
reduction and morphology control, some novelty in terms of alternative pathway or
modified parameters may need to be imposed into the conventional synthesis method.
In the present PhD studies, research activities have been focused on the synthesis of
Co-layered hydroxide compounds as precursors for related nanostructured materials.
The synthesis methods involved are mainly precipitation at ambient conditions.
The literature review in this chapter covers three parts: i) a detailed review of
layered hydroxide materials, mainly hydrotalcite-like compounds; ii) a particular
discussion about cobalt-layered hydroxides; and iii) a brief introduction of
nanostructured materials.